Transform List into Map using only two keys and odd or even list indexes as values - Java 8 Stream

You were on the right track with streaming over the indexes:

import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.mapping;
import static java.util.stream.Collectors.toList;

IntStream.range(0,list.size())
        .boxed()
        .collect(groupingBy(
                i -> i % 2 == 0 ? "even" : "odd", 
                mapping(list::get, toList())
        ));

If you are ok with having your map be indexed by a boolean you can use partitioningBy:

IntStream.range(0, list.size())
        .boxed()
        .collect(partitioningBy(
                i -> i % 2 == 0, 
                mapping(list::get, toList())
        ));